1. Field of the Invention (Technical Field)
The present invention relates to methods and vascular assist apparatuses for preventing the development of venous thrombosis of the lower extremities.
2. Description of Related Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Lower extremity venous thrombosis and pulmonary embolism is a significant cause of mortality and morbidity in patients. The condition arises as a result of inadequate circulation in sedentary, hospitalized patients leading to stagnation of blood and formation of thrombi in the veins, particularly in the venous sinuses and valve cusp areas.
Sequential and non sequential compression devices are known to alleviate the condition. Generally, these devices employ a short period of compression followed by a cycle of decompression to increase flow in the veins. For example, in one device comprising an extremity garment, chambers within the garment are sequentially inflated from ankle to knee (or mid thigh) to a maximum pressure of 45–50 mm Hg at the ankle, 35 mm Hg at the calf, and 30 mm at the thigh. Cycles of compression followed by relaxation are employed, such as a duration of compression of 11 seconds with a 60-second relaxation period between compressions.
Several patents disclose devices and methods to assist vascular circulation. For example, U.S. Pat. No. 6,007,559 discloses a “vascular assist device” in which statis pressure is graduated, such as from the ankle to the hip. In one embodiment, there are two separate cuffs.
U.S. Pat. No. 5,117,812 discloses separate “segments” for applying pressure about the leg in an inflation or compression cycle applying pressure first to the distal segment of the leg followed by pressure to the proximal segment. U.S. Pat. No. 5,014,681 discloses an intermittent pressure sleeve. U.S. Pat. No. 4,841,956 discloses a two-compartment cuff, with a proximal calf pump operated first followed by operation of the distal calf pump. Other patents disclosing compression devices include U.S. Pat. Nos. 4,865,020, 5,022,387, 5,109,832, 5,186,163, 6,231,532 and 6,440,093.
U.S. Pat. No. 2,140,898 discloses the use of a cuff positioned at the upper thigh to intermittently apply pressure and restrict return blood flow. The stated result is that an active vaso-dilation of the arteries occurs causing a stated increase in arterial circulation.
The prior art is directed at stimulating fluid flow in an attempt to move blood in a manner that will prevent the formation of thrombi. Some devices, such as sequential-TEDS devices, such as those sold by Kendall Co., are marketed and used to achieve sequential compression, by sequentially compressing a portion of the leg, such as the calf and/or lower thigh, in a sequential manner starting at the most distal point and sequentially proceeding to the proximal. Other devices, such as the PlexiPulse® pneumatic compression device (KCI USA), apply non-sequential compression, such as to the foot and/or calf. However, in many patients, refill of the veins is inefficient, and none of the prior art addresses the problem such inefficient refill presents in allowing blood to remain stagnant in various parts of the venous system—particularly in the sinuses and valve cusps. In such patients, the blood pooling or stasis is such that the pressure produced by devices of the prior art cannot alone fully flush the venous system. Therefore, there is a need for devices and methods to fully flush the venous system, particularly the valve cusp areas where thrombus usually originates and blood flow is the least.